docs(import[gitlab]) Add subgroup targeting and workspace structure table

why: Live testing of all permutations revealed two undocumented behaviors:
slash-notation for direct subgroup targeting and the workspace nesting
rules relative to target depth.

what:
- Add "Subgroup targeting" section with slash notation examples
- Add workspace structure table covering all target/flatten combinations
- Clarify that --flatten-groups is a no-op when target is already a leaf

Author: tony

docs/cli/import/gitlab.md

@@ -14,6 +14,24 @@ Import repositories from GitLab or a self-hosted GitLab instance.
     :path: import gitlab
 ```
 
+## Subgroup targeting
+
+Use slash notation to target a specific subgroup or sub-subgroup directly:
+
+```console
+$ vcspull import gl my-group/my-subgroup \
+    --mode org \
+    --workspace ~/code/
+```
+
+```console
+$ vcspull import gl my-group/my-subgroup/my-leaf \
+    --mode org \
+    --workspace ~/code/
+```
+
+The `TARGET` argument accepts any depth of slash-separated group path.
+
 ## Group flattening
 
 When importing a GitLab group with `--mode org`, vcspull preserves subgroup
@@ -27,6 +45,22 @@ $ vcspull import gl my-group \
     --flatten-groups
 ```
 
+### Workspace structure by target and flag
+
+Given a group tree `my-group → sub → leaf`, importing from `~/code/`:
+
+| Target | `--flatten-groups` | Workspace sections written |
+|--------|:-----------------:|---------------------------|
+| `my-group` | no | `~/code/`, `~/code/sub/`, `~/code/sub/leaf/` |
+| `my-group` | yes | `~/code/` only |
+| `my-group/sub` | no | `~/code/`, `~/code/leaf/` |
+| `my-group/sub` | yes | `~/code/` only |
+| `my-group/sub/leaf` | no | `~/code/` only (leaf — no further nesting) |
+| `my-group/sub/leaf` | yes | `~/code/` only |
+
+When the target is already the deepest group (a leaf), `--flatten-groups` has
+no effect — all repositories already land in the base workspace.
+
 ## Authentication
 
 - **Env vars**: `GITLAB_TOKEN` (primary), `GL_TOKEN` (fallback)